Various embodiments of actuators of the type described above are known in the art. Reference can be made to e.g. the designs described in the documents DE 25 18 681, DK 156 745, SE 379 404 and U.S. Pat. No. 3,498,187. Typical features of these constructions are that they are comparatively complicated and require quite a number of components and machining processes, which makes the product expensive. Nevertheless, all necessary details have not been shown in these patent documents, e.g. radially directed grooves in the bearings for the shaft in order to secure the shaft, details which are conventional but which require expensive machining processes. This concerns at least the constructions described in DE 25 18 681, DK 156 745 and U.S. Pat. No. 3,498,187. A special feature of the construction according to SE 379 404 is that the piston area is relatively small as compared to the piston stroke, because the teeth of the gear ring segment are rods in a groove, with which the rack meshes. The torsional moment, which the actuator can generate, is consequently relatively small in relation to the overall dimensions of the device.